The present invention relates to golf ball materials having heat resistance, flowability and processability, providing high-performance golf balls having excellent properties such as rebound and durability. The invention also relates to golf balls comprising as an essential component therein a molding made from such a golf ball material, and to methods for preparing such golf ball materials.
In recent years, ionomer resins have been widely used as cover materials of golf balls. Ionomer resins are ionic copolymers comprising an olefin such as ethylene and an unsaturated carboxylic acid such as acrylic acid, methacrylic or maleic acid, having some of the acid groups neutralized with metal cations such as sodium, lithium, zinc or magnesium. These resins provide excellent characteristics such as durability, rebound resilience and scuff resistance of the ball.
Today, the base resins used as cover materials of golf balls are mostly ionomer resins, but a variety of modifications are being made to match the constant desire by golfers for golf balls having a high rebound resilience and an excellent carry-and-run performance.
For example, to improve the rebound resilience and to reduce the cost of ionomer cover materials, U.S. Pat. No. 5,312,857, U.S. Pat. No. 5,306,760 and International Application WO 98/46671 describe cover materials composed of ionomer resins with a large amount of metallic soap added.
However, the metallic soap in such cover materials undergoes decomposition and vaporization during injection molding, releasing a large amount of fatty acid vapor, easily causing molding defects. The released vapor coagulates to form deposits on the surface of the molding, markedly lowering its paintability. The rebound resilience of these cover materials does not differ to any considerable extent from the rebound resilience of the ionomer covers of the same hardness containing no metallic soap; either the rebound resilience in both cases is almost the same or, at most, only a small positive effect on the rebound resilience is observable from the metallic soap composition.
Such a composition therefore does not markedly improve the rebound resilience. Moreover, depending on the type of metallic soap used, the processability and the rebound resilience are sometimes greatly lost, making the cover material far off the level of practical use.
An ionomer for use as a golf ball material has recently been developed in the form of a high rebound resilience material having an interpenetrating network (IPN) structure and a homogeneous phase. The ionomer is obtained by mixing a first component such as an ethylene-(meth)acrylic acid copolymer with a different type of thermoplastic resin as a second component to form a resin composition, and then adding a metal ionic species as a third component to neutralize the acid groups in the first component melt-mixed in the resin composition (U.S. Patent Application Publication No. 2004/0044136). However, in this prior-art production method, as a metal ionic species such as a metal oxide, metal hydroxide or metal carbonate is used directly in the form of a solid (powder or granular material), particularly in some case of using a large amount of the solid due to a high degree of neutralization, a poor dispersion may arise during the melt-mixing, leaving some of the metal ions unreacted. There is a concern that such unreacted metal ions may deteriorate the physical properties of the ionomer material obtained.
From the viewpoint of the above described, the objects of the invention concerning no direct use of a metal ionic species such as a metal oxide, a metal hydroxide or an acid metal salt are:    1) providing golf ball materials having good properties such as heat resistance, flowability and processability by use of which high-performance golf balls having excellent properties such as durability, scuff resistance and optimal hardness are obtained with a minor sacrifice of rebound resilience,    2) providing golf balls comprising a molding made from such a golf ball material as an essential component,    3) providing a method for preparing such golf ball materials.